Spatial Audio Processing: MPEG Surround and Other Applications

4.4 Coding of Low-frequency Effects (LFE) Audio Channels

Commonly used multi-channel surround formats, such as 5.1 surround (Section 2.2.3), use LFE channels. An LFE channel, as defined for the 5.1 standard [150], contains only frequencies up to 120 Hz. In the following the incorporation of the LFE channel in BCC is described. The same principles are applicable to other surround formats.

At frequencies below 120 Hz, six-channel BCC is applied, i.e. all six channels including the LFE channel are coded. At frequencies above 120 Hz, five-channel BCC is applied, i.e. all channels except the LFE channel. The LFE channel is not considered at higher frequencies since it does not contain any signal energy there.

This is implemented specifically by using a filterbank with a lowest sub-band covering 0–120 Hz. For this lowest sub-band the LFE channel is considered and for all other sub-bands the LFE channel is ignored. BCC only considering ICLD is applied to this lowest frequency sub-band since at such low frequencies spatial hearing is limited and the purpose of applying BCC is merely to provide each loudspeaker with the same power as was present in the original audio signal.

Since LFE channels are considered only in the lowest frequency BCC sub-band, at each time instant only a single ICLD parameter per LFE channels is used. Thus the amount of side information does not notably increase by including one or two LFE channels.